The present invention is directed to vehicle suspension systems, and in particular to suspension systems incorporating interconnected dampers and active roll control.
not applicable
not applicable
not applicable
In order to provide increased ride comfort and vehicle stability, a wide variety of solutions have been proposed. These can be categorised into several groups; damping systems; passive roll control and vehicle support; active roll control systems; active body control systems (also known as low-bandwidth active systems); and fully active (high bandwidth) suspension systems.
Damping systems proposed cover a wide range of concepts from passive, single wheel dampers (which provide damping forces determined in part by: position in the stroke, acceleration of the damper piston; load on the wheel) through passive interconnected dampers (which can provide different damping forces for the different suspension modes of roll, pitch and heave) up to electronically controlled xe2x80x9csemi-activexe2x80x9d dampers which modulate the damping force in dependence on sensed vehicle operating conditions (such as roll, yaw, etc.) to enable optimal damping to be provided at all times, giving increased comfort and stability. Without providing heave or roll stiffness, there is a limit to how effective these type of systems can be.
Passive roll control and vehicle support systems which decouple different suspension modes can vary from completely mechanical systems through to completely hydraulic systems. The ability to decouple the roll, warp and heave stiffness modes of a suspension allows more optimal tuning of each stiffness, allowing increased roll stiffness and stability and increased comfort. However, there is a limit to the amount of roll stiffness which can be provided before the ride comfort deteriorates. There is a trend towards suspension systems with very low roll angles, even at large lateral g, which to achieve passively can require excessive roll stiffness, thereby limiting ride comfort.
Active roll control systems usually rely on lateral stabiliser bars with hydraulic actuation to enable control of body roll angle through controlling the torsional force in the stabiliser bar. Packaging of the stabiliser bar including the hydraulic actuator can be difficult, especially at the front of a vehicle around the front subframe, suspension geometry and engine.
Active body control systems generally use conventional, soft springs for each wheel to improve comfort in conjunction with hydraulic actuators to control and limit body motion in roll and pitch. As these systems are controlling only body position, they do not need to respond to high frequency, small magnitude individual wheel inputs, which are generally absorbed by the low spring rate. The controllers can therefore be relatively low speed or xe2x80x9clow bandwidthxe2x80x9d. These types of systems do not generally provide vehicle damping.
Fully active suspension systems support the vehicle on hydraulic struts which are all controlled by a central computer. In order to react fast enough each wheel may have a local fluid source and a local computer to control small high frequency (high bandwidth) wheel inputs. The local computer usually senses actuator load and position, wheel and body acceleration, etc. This must communicate with the central computer which controls overall body position by communicating with the computers for each wheel and using additional inputs such as throttle, brake and steering positions, body accelerations, actuator positions. These systems are highly complex and expensive.
Most of the above types of suspension system have reached only limited production, due to varying reasons such as conflicts between cost, packagability, complexity, efficiency, weight and refinement
It is therefore preferable to provide a suspension system incorporating an interconnected damper arrangement providing damping and roll stiffness and further incorporating a low bandwidth active control of the vehicle roll angle.
It is therefore an object of the present invention to provide a vehicle suspension system incorporating an active roll control system including at least four wheel cylinders.
It is another object of the present invention to provide a combined damping and active roll control system.
With this in mind, the present invention provides a damping and roll control system for a vehicle suspension system, the vehicle having at least one pair of laterally spaced front wheel assemblies and at least one pair of laterally spaced rear wheel assemblies, each wheel assembly including a wheel and a wheel mounting locating the wheel to permit movement of the wheel in a generally vertical direction relative to a body of the vehicle, and vehicle support means for providing at least substantially all of the support for the vehicle; the damping and roll control system including;
wheel cylinders respectively locatable between each wheel mounting and the body of the vehicle, each wheel cylinder including an inner volume separated into first and second chambers by a piston supported within the wheel cylinder;
first and second fluid circuits respectively providing fluid communication between the wheel cylinders by fluid conduits, each said fluid circuit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle and the second chambers of the wheel cylinders on the opposite side of the vehicle to thereby provide roll support decoupled from the warp mode of the vehicle suspension system by providing a roll stiffness about a level roll attitude whilst simultaneously providing substantially zero warp stiffness;
each fluid circuit including one or more fluid accumulators for providing roll resilience;
the or at least one of the accumulators on each fluid circuit including an accumulator damper means for controlling the rate of fluid flow into and out of the accumulator;
damper means for controlling the rate of fluid flow into and out of at least one chamber of each wheel cylinder; and
a fluid control means connected to said first and second fluid circuits for supplying or drawing fluid from each said fluid circuit as a function of the ride characteristics of the vehicle;
the damping and roll control system thereby providing substantially all of the damping of the vehicle suspension system.
The damper means of the damping and roll control system are being provided to control the rate of fluid flow into and out of at least one chamber of each wheel cylinder to thereby provide substantially all of the damping of the vehicle suspension system. The conduits may be sized to provide at least a portion of the high speed damping of the vehicle suspension system, but as they give a fixed, non-linear effects normally damper means are also necessary,
The vehicle support means may in certain embodiments of the present invention provide at least substantially all of the support for the vehicle.
The damping and roll control system therefore provides damping for the vehicle suspension and provides a roll stiffness without introducing a corresponding warp stiffness.
Each fluid circuit may in one preferred embodiment according to the present invention include a first fluid conduit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle; and a second fluid conduit providing fluid communication between the second chambers of the wheel cylinders on the opposite side of the vehicle; the first and second fluid conduits being in fluid communication.
According to another preferred embodiment according to the present invention, each fluid circuit may include first and second diagonal fluid conduits, each respectively providing fluid communication between the first chamber of one wheel cylinder on one side of the vehicle and the second chamber of the diagonally opposite wheel cylinder on the other side of the vehicle; the first diagonal fluid conduit between one pair of diagonally opposite wheel cylinders being in fluid communication with the second diagonal fluid conduit between the other pair of diagonally opposite wheel cylinders.
According to yet another preferred embodiment according to the present invention, each fluid circuit may include a front fluid conduit providing fluid communication between the wheel cylinders of the front wheel assemblies, and a rear fluid conduit providing fluid communication between the wheel cylinders of the rear wheel assemblies, with the front and rear conduits respectively providing fluid communication between the first chamber of the wheel cylinder at one side of the vehicle with the second chamber of the wheel cylinder at the opposite side of the vehicle, the front and rear conduits being in fluid communication.
It is to be appreciated that other connection arrangements are also envisaged. It is also to be appreciated that the same principles may be applied to vehicles with more than four wheels. For example, to apply the system to a six wheeled vehicle, the additional left hand wheel cylinder will have its first chamber connected to the conduit connecting the first chambers of the other two left hand wheel cylinders, and its second chamber connected to the conduit connecting the second chambers of the other two left hand wheel cylinders. The connection of the other cylinder to the right hand side of the vehicle similarly communicates first chambers together and second chambers together.
The damper means may be located at or in the wheel cylinders, in the conduits, and/or in a manifold block. The manifold block may be centrally located in the vehicle and may provide the required fluid communication between the first and second conduits to form the first and second fluid circuits. The damper means may be bi-directional (ie. provide controlled flow restriction in both directions), in which case each wheel cylinder may require only one damper means for one of the first or second chambers. In this case, the associated chamber may try to suck a vacuum it the damper valve is not supplying fluid at the same rate at it is being demanded. This can lead to aeration of the fluid and potential loss of ride control by the system. To avoid this effect, a single direction damper valve may be used to ensure that the wheel cylinder chambers only act through a damper valve when expelling fluid, thereby substantially preventing fluid aeration in the cylinder chambers. Alternatively, the single direction damper valve may be used in parallel with a non-return valve. Alternatively, to provide large damping forces with reliable, compact damper valve means, a bi-directional damper means may be provided for each of the first and second chambers of at least one pair of laterally spaced wheel cylinders.
Each said fluid circuit, includes a primary fluid accumulator to allow for changes in the fluid volume of each circuit to thereby provide roll resilience. Also, if a wheel cylinder with differing effective piston areas between the first and second chambers is used (for example a piston having a rod extending from one side only, as in a conventional damper cylinder assembly), the accumulator needs to be able to accommodate the rod volume changes within the system during bounce motions of the suspension. In this case, in roll, the accumulator absorbs a much greater change of fluid volume per unit displacement of the wheel cylinders than it absorbs in bounce as both the effective areas of a first chamber side and a second chamber side are working to displace fluid into the accumulator giving a correspondingly higher stiffness for roll motions of the roll Control system than for bounce motions.
Each fluid circuit may include at least one secondary fluid accumulator to provide increased roll resilience. Between each second accumulator and the respective fluid circuit there may be a roll resilience switching valve. When the vehicle is travelling in a straight line, the valve may be held open to allow the second accumulators to communicate with the associated fluid circuits to provide additional roll resilience, thereby further improving ride comfort. When turning of the vehicle is detected, the roll resilience switching valve is closed to provide a desirable increase in roll stiffness during cornering. The detection of vehicle cornering may be performed in any known manner, using inputs for conditions such as steering rate of change, steering angle, lateral acceleration and vehicle speed. Any or all of these sensors and/or others not cited may be used,
The accumulators may be of the gas or mechanically sprung piston type or the diaphragm type and either or both can be beneficial in limiting pressure change within the system due to fluid expansion and contraction with temperature changes, and in increasing the time to maintenance of the system by replenishing fluid lost from the system through leaks past rod seats and out of fittings. Any fluid loss should be minimal, therefore the effect on the operating pressure of the system over time may be negligible.
At least one of the accumulators in each fluid circuit has a damper means to control the rate of fluid flow into and/or out of the accumulator. Due to the higher rate of fluid flow into and out of the accumulators in roll when compared to bounce (as discussed earlier), the effect of the accumulator dampers is greater in roll than in bounce giving a desirable high roll damping to bounce damping ratio. It the accumulators are not damped, the roll damping is determined by the bounce damping, as is the case when using conventional dampers. Damping the accumulators (which provide roll resilience in the system) is also preferable when providing active control of the roll attitude of the vehicle, improving system response and reducing overshoot in the control system.
Damping the accumulators can also have a detrimental effect to single wheel input harshness as single wheel inputs are also heavily damped by accumulator dampers. To increase comfort in straight line running, it can therefore be advantageous to provide a bypass passage around the accumulator damper valve to permit fluid to bypass the damper for at least one accumulator. The bypass passage includes a valve to open or close the passage. During turning, the valve is in the closed position and the accumulator damper valves are providing high roll damping. In straight line running, the valve is open to reduce the roll and single wheel input damping forces in the system.
The roll control system may have a pressure precharge to allow the accumulators to function and supply fluid in rebound motions of the wheels (where they fall away from the vehicle body). This precharge is preferably about bar for the roll control system with the vehicle at standard unladen ride height
It may be preferable to use a wheel cylinder design with a rod protruding from one side of the piston through only one chamber. This allows for a simple and cheap cylinder design, but any system precharge pressure acting over the unequal effective piston areas in the first and second chambers produces a net cylinder force. This force may provide some support of the vehicle body although the proportion of vehicle load supported by the roll control system is usually very small and is generally a similarly negligible percentage of (although it may be more than) the degree of support provided by a conventional precharged damper cylinder assembly. The exact amount is determined by the cylinder rod and bore dimensions, system precharge pressure and cylinder to wheel hub lever ratio.
For example, in the case where the first chamber of each wheel cylinder is in compression as the wheels move upwardly with respect to the vehicle body, and the effective area of the piston on the first chamber side is larger than the effective area of said piston on the second chamber side, thereby providing a degree of support of the vehicle body.
If accumulators with a non-linear spring function (ie a hydropneumatic accumulator which has an increasing stiffness in compression and a decreasing stiffness in rebound) are used and the roll control system provides a degree of vehicle support (as outlined above), then as the vehicle rolls due to lateral acceleration, the total volume of fluid in the accumulators can decrease overall, increasing the fluid volume in the roll control system and causing an overall increase in vehicle height (known as xe2x80x9croll jackingxe2x80x9d). The degree of vehicle support provided by the roll control system influences the degree of roll jacking, with the effect being minimal when the roll system supports a low proportion of vehicle load.
It may be desirable to produce the inverse of the roll jacking effect such that the average height of the vehicle is lowered during cornering. This effect can be produced in the case where the first chamber of each wheel cylinder is in compression as the wheels move upwardly with respect to the vehicle body, and the effective area of the piston on the second chamber side is larger than the effective area of said piston on the first chamber side, thereby providing a degree of additional load on the vehicle support means, tending to push the vehicle down towards the ground.
Preferably, a simpler arrangement may be used with the cheaper cylinder design which provides vehicle support (discussed above). The resilient means in the first accumulator may include one or more mechanical springs such that the spring rate in the compression direction from the normal static position is lower than the spring rate in the rebound direction from the normal static position, to thereby give the reverse effect of a conventional hydropneumatic accumulator and lower the average height of the vehicle during cornering. Additionally or alternatively, the rebound damping rate of the accumulators may be higher than the compression damping rate to provide a similar vehicle lowering effect and better response to steering inputs during initial cornering (turn-in). Indeed, only rebound damping may be provided for the accumulators, with a non-return valve allowing virtually unrestricted flow in the compression direction (although this may not always be desirable as there can be side effects in the response of the active roll control).
Ideally, the roll control system should not provide any vertical support of the vehicle. Therefore, in another, preferred embodiment of the present invention, the effective piston areas in the first and second chambers of each cylinder may be similar, the roll control system thereby supporting substantially zero vehicle load. As the amount of vehicle load support provided by the roll control system is one of the main factors controlling the amount of roll jacking inherent in the system, using wheel cylinders with similar effective piston areas in the first and second chambers and which therefore do not provide any vehicle support provides the roll control system with zero roll jacking.
However, in some applications, the use of a cylinder having piston rods extending from both ends thereof can lead to packaging difficulties because of the need to provide clearance for the upwardly extending piston rod. Therefore, according to another preferred embodiment, a piston rod may extend from one side of the piston, the piston rod having as small a diameter as physically possible to minimize the vehicle support provided by the damping and roll control system. In another possible arrangement, a hollow piston rod may extend from one side of the piston, and an inner rod may be supported within the inner volume of the cylinder, the inner rod being at least partially accommodated within the hollow piston rod, the hollow piston rod moving together with the piston relative to the inner rod. This arrangement also acts to minimize the difference in area of the opposing piston faces to minimize the vehicle support provided by the damping and roll control system. This arrangement can also be designed to provide a smaller effective piston area on the compression chamber (of the roll control pair of chambers in the cylinder) than on the rebound side to give a lowering of the vehicle in roll, as described above.
According to an alternative preferred embodiment, the hollow piston rod arrangement of the wheel cylinder may be adapted to also provide a vertical support function for the vehicle. The piston supported in the wheel cylinder may provide an upper and lower chamber. The inner rod when supported within the hollow piston rod defines a rod chamber. This rod chamber may be used as part of a fluid circuit of the roll control system. To this end, the area of the peripheral end of the inner rod may be at least substantially identical to (or greater than) the area of the piston facing the lower chamber. The upper chamber may be sealed to provide a bounce chamber to provide resilient support for the vehicle. The rod chamber, together with the lower chamber, form a respective part of a fluid circuit of the roll control chamber.
It should be noted that the roll moment distribution for the roll control system is determined by the ratio between the effective piston areas of the front wheel cylinders compared to the effective piston areas of the rear wheel cylinders. Ideally, in most applications, each wheel cylinder should have a constant ratio between the effective piston area on the first chamber side compared to the second chamber side.
One advantage of using cylinders where the piston rod is only provided extending from the one piston face is that the degree of support provided by the cylinders can be varied by varying the support height of the vehicle. As the vehicle is lowered the support provided by the roll control system increases leading to higher roll stiffness. This is an affect of having an increased volume of piston rod introduced into the roll control system.
The support means for at least one pair of laterally spaced wheel assemblies may include first support means which are independent for each wheel assembly, thereby contributing an additional roll stiffness to the suspension system. Both the vehicle support means and the roll control system can together provide the roll stiffness for the vehicle in this arrangement
Additionally or alternatively, the support means for at least one pair of laterally spaced wheels may include second support means which are interconnected between each wheel thereby contributing substantially zero roll stiffness to the suspension system. This and other vehicle support arrangements that provide little to no roll support and combinations of support arrangements are described in the Applicants"" International Application No. PCT/AU97/00870 referred to previously. In such an arrangement, the damping and roll control system can provide substantially all of the roll control for the vehicle. Furthermore, if the support means have substantially zero roll stiffness, the damping and roll control system can provide substantially all of the roll control for the vehicle. In this case, neither the support means or roll control system provides significant warp stiffness. This allows for substantially free warp motion of the vehicle wheel assemblies, improving comfort, reactions to single wheel inputs and providing substantially constant wheel loads (and therefore improved traction) in low speed or non-dynamic warp motions when traversing uneven terrain such as in off-road situations.
In a second preferred embodiment of the present invention, the first and second fluid circuits may be in fluid communication such that fluid may be transferred therebetween. To this end, at least one bridge passage may interconnect the first and second fluid circuits to provide for said fluid communication. The bridge passage may be provided by a bridge conduit. Alternatively, the bridge passage may be provided within a connector body to which the conduits of the first and second circuits are connected. At least one flow control valve may be provided for controlling the flow through the bridge passage. However, the bridge passage may be inappropriate for use in the roll control system when the total volume of fluid in the system is fixed (there is no external reservoir). In many cases it may only be useful for system setup.
One or more accumulators may optionally also be provided for the bridge passage. The flow control valve and accumulator may be provided on a said bridge conduit. According to another preferred embodiment, the control valve and/or accumulator may be supported on the connector body. It is also possible for all the damper valves and accumulators previously referred to be located on a common said connector body to simplify the packaging of the system within a vehicle.
The flow control valve may be opened, for example when there is little demand on the roll control system when the vehicle is travelling on a straight road. When the flow control valve is opened, this leads to a xe2x80x9cshort-circuitingxe2x80x9d of the system such that the first and second chambers of each cylinder are allowed to communicate directly.
The operation of the flow control valve may be controlled by an Electronic Control Unit on the basis of operational parameters such as the lateral acceleration, speed and steering rate of the vehicle.
It is also possible for a plurality of bridge passages to be provided interconnecting the first and second fluid circuits. Each bridge passage may be provided with a said flow control valve.
It is also possible that the wheel cylinder include an integral flow control valve and/or damper valve therein. The piston of the wheel cylinder may include a flow control valve and/or damper valve controlling the flow of fluid between the first and second chambers.
The use of a plurality of bridge passages having flow control valves or wheel cylinders having built-in flow control valves facilitates fluid flow between the first and second chambers of the wheel cylinders. This can lead to a reduction in the inertia forces due to fluid flow through the system resulting in improved isolation of high frequency inputs and sharp edge inputs to the vehicle wheels. The effect of inertia forces within the roll control system will be subsequently described in more detail.
In the second preferred embodiment of the invention, as the roll control system can be switched to provide substantially zero roll stiffness, the use of zero roll stiffness support means for all wheels is not viable. However, zero roll stiffness support means may still be used in combination with independent support means providing some roll stiffness. Therefore, the support means for at least one pair of laterally spaced wheels may include first support means for supporting at least a portion of the load on the associated wheel assemblies, said first support means providing independent resilience for each respective wheel and thereby providing a roll stiffness.
Additionally, the support means for at least one pair of laterally spaced wheels may include second support means for supporting at least a portion of the load on the associated wheel assemblies, said second support means providing combined resilience for each associated wheel assembly, equalising the support force provided to the wheel assemblies and thereby providing substantially zero roll stiffness.
The fluid control means supplies or draws fluid from the fluid conduits of each fluid circuit in one preferred embodiment, the fluid control means may be connected to the first and second fluid circuits so that fluid can be drawn from one fluid circuit while fluid is at the same time pumped into the other fluid circuit. The fluid control means may be controlled by means of an Electronic Control Unit that can, for example, utilize signals indicating vehicle speed, body accelerations, the rate of change of the steering angle and the wheel positions to determine the ride characteristics of the vehicle at any particular instance.
The fluid control means may for example include a fluid pump and a valve for controlling the interconnection of the fluid circuits with the pump. The valve may be solenoid actuated and may be controlled by the Electronic Control Unit. The use of low-pressure fluid means that a relatively high volume of fluid is contained within the roll control system according to the present invention when compared with systems using high-pressure fluid. The practical effect of this is that a relatively high volume of fluid needs to flow through the roll control system when handling different wheel inputs and vehicle motions.
The fluid control means previously described can be used to compensate for the large fluid flows in each fluid circuit by providing additional fluid to one of the fluid circuits or by allowing a transfer of fluid between the fluid circuits to thereby maintain the fluid volume of each fluid circuit. This however requires that the fluid pump of such a system to be suitably oversized to handle the anticipated high fluid flow. The resultant component and operational costs can therefore be quite expensive.
Therefore, according to another preferred embodiment, the fluid control means may further include a fluid volume control unit for supplying and withdrawing fluid from each fluid circuit. The fluid volume control unit may have an inner volume, with a piston assembly being slidably supported within the inner volume. The inner volume may be in the form of a cylindrical bore having a generally cylindrical wall. The piston assembly and inner volume may define two supply chambers provided adjacent opposing sides of the piston assembly such that movement of the piston assembly in any one direction will result in a corresponding reduction and expansion in volume of the respective supply chambers. Each supply chamber may be in fluid communication with one of the fluid circuits of the roll control system, for example, by means of a fluid supply line.
The fluid volume control unit may also include actuation means for displacing the piston assembly. According to a preferred embodiment, the actuation means may be a high pressure hydraulic fluid supply system having at least two fluid outlets. The piston assembly may include a pair of pistons coupled by a common shaft. A central separation wall may be provided within the inner volume of the fluid supply unit, with the common piston shaft extending through an aperture within that wall. The piston shaft may have a relatively wide diameter so as to substantially fill the section of the inner volume in which the piston shaft is located thereby leaving a relatively narrow cylindrical cavity between the inner volume wall and the shaft. The central separation wall may separate this cavity into two actuation chambers.
A separate fluid inlet may be provided for each actuation chamber, and each fluid inlet may be respectively connected to one of the fluid outlets of the hydraulic fluid supply system. The piston assembly may therefore be moved by respectively supplying and removing hydraulic fluid from the actuation chambers. The actuation chambers and hydraulic fluid supply system may provide a high pressure side of the arrangement. The supply chambers and fluid circuits of the roll control system may together provide a low pressure side of the arrangement. The total volume of the supply chambers is substantially greater than the total volume of the actuation chambers. Therefore, the volume of fluid transferred between the supply chambers and the fluid circuits is substantially greater than the volume of hydraulic fluid transferred through the actuation chambers by the hydraulic fluid supply system. This allows a relatively small hydraulic fluid supply system to be used lo actuate the fluid volume control unit. The fluid volume control unit therefore acts as a xe2x80x9cvolume amplifierxe2x80x9d for the hydraulic fluid supply system.
It a fluid volume control unit and a bridge valve are both provided, it may be preferable to add an additional pressure maintenance valve such that the high pressure hydraulic fluid supply system can be used to regulate the pressure in the first and second fluid circuits. Periodically, when the vehicle is travelling in a straight line, the high pressure hydraulic fluid supply system may be regulated to the desired precharge pressure for the first and second fluid circuits, then the pressure maintenance and bridge valves may be momentarily opened. The opening and closing of these valves may be variably controlled (such as by pulse width modulation) to reduce harshness and improve refinement. It is also envisaged that mechanical actuation means may be used to displace the piston assembly of the fluid volume control unit. For example, the piston assembly may include a piston shaft extending outside of the inner volume, with the piston shaft engaging and being driven by a worm gear or other type of mechanical or electric drive. Other actuation means are however envisaged.
The fluid volume control unit operates in effect by providing a volume of fluid to a fluid circuit requiring additional fluid, while simultaneously venting the same volume of fluid from the other fluid circuit. This ensures that the same general volume of fluid is maintained in each fluid circuit. Furthermore, it ensures that the system pressure is maintained within the two fluid circuits. Furthermore, the accumulators of each fluid circuit can always operate properly under various wheel and vehicle operational conditions. Known active roll control systems completely vent all pressure from the unloaded system in roll, rather than merely transferring fluid between the fluid circuits. The result of maintaining pressure in both circuits is an improved response time for the roll control system.
Another advantage of using a fluid volume control unit as described above is that the roll control system could be a low pressure pneumatic system, with gas being used within the fluid circuits and wheel rams. The accumulators could then be in the form of a gas reservoir. This is because the fluid volume control unit will separate the high pressure hydraulic section of the roll control system from the low pressure pneumatic section. The advantage of using a pneumatic arrangement is that it reduces the production of mass flow effects in the roll control system because of the low mass of the gas flowing through the system Ride harshness can be significantly reduced due to the inherent compressibility of the gas which allows small, high frequency inputs to be at least partially absorbed without requiring flow through the whole system.
It is to be appreciated that the conduit size may be selected to provide a degree of the damping required by the damping and roll control system. Depending on the level of ride comfort required in an application, the conduit size may be selected based on a variety of factors such as fluid inertia, fluid friction due to viscosity through range of operating temperatures, etc.
The vehicle support means preferably provides most if not all the vertical support for the vehicle. The damping and roll control system however preferably provides little to no vertical support for the vehicle so the operating pressure can be lower than that required for actively controlled support systems for example. Although the wheel cylinders can experience high dynamic pressures, the pressures are still low enough that existing damper technologies may be applied, Additionally, the majority of fluid conduits in the system may experience lower peak pressures than the wheel cylinders (depending on the position of damper valving) allowing the use of cheaper, lower rated components.
The damping and roll control system of the suspension system according to the present invention can therefore use low pressure components. The wheel cylinders can be constructed using standard vehicle damper and sealing technology. This leads to substantial manufacturing cost savings when compared to higher pressure systems. Also, comfort and NVH problems associated with higher pressure systems such as xe2x80x9cstictionxe2x80x9d between components are minimized in low pressure systems, the stiction levels being similar to that present in a conventional damper cylinder assembly.
Such a damping and roll control system can be installed in existing vehicle suspension systems, the dampers used in such systems being replaced or adapted for use as the wheel cylinders of the roll control system according to the present invention. The existing vehicle support means supporting the vehicle such as conventional steel or pneumatic springs can be retained. Alternatively, the vehicle support means may be replaced by support means that provide little to no roll support as described previously. This is possible because the damping and roll control system also provides a roll stiffness for the vehicle suspension system.
It will be convenient to further describe the present invention with respect to the accompanying drawings which illustrate preferred embodiments of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.